In a mass spectrometer using an atmospheric pressure ion source, such as an electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) or atmospheric pressure photoionization (APPI), the ionization chamber is maintained at a substantially atmospheric pressure, whereas the analysis chamber, which contains a mass separator (e.g. quadrupole mass filter) and an ion detector, needs to be maintained at a high degree of vacuum. Therefore, in such a mass spectrometer, the structure of a multistage differential pumping system is normally used, in which one or more intermediate vacuum chambers are provided between the ionization chamber and the analysis chamber so as to increase the degree of vacuum in a stepwise manner. In such a mass spectrometer having the structure of the multistage differential pumping system, an ion transport optical system, which may also be called the “ion lens” or “ion guide”, is arranged within each intermediate vacuum chamber. An ion transport optical system is a kind of optical device for transporting ions to the rear stage while focusing those ions (and accelerating or decelerating them in some cases) by the effect of a direct-current electric field, a radio-frequency electric field, or both of these two electric fields.
In order to transport ions while efficiently trapping them, ion transport optical systems with various structures and configurations have been used in conventional practices. In one widely used specific form of the ion transport optical system, multiple electrodes are provided around or along the ion beam axis, and two radio-frequency voltage whose phases are inverted from each other by 180 degrees are applied to any two electrodes neighboring each other among those electrodes. Owing to the effect of the radio frequency electric field created by the radio-frequency voltages, ions are transported while being trapped or focused. Representative examples of this specific form of ion transport optical system include: a multipole ion guide in which an even number of rod electrodes equal to or more than four are arranged around the ion beam axis; and a multipole array ion guide in which virtual rod electrodes, each of which consists of a number of plate electrodes arranged in the direction of the ion beam axis, are used in place of the normal rod electrodes.
Patent Literature 1 discloses an ion transport optical system having a structure called an “ion funnel”, in which a number of aperture electrodes each of which has a circular aperture are arranged along a traveling direction of ions, where the area of the apertures gradually decreases. The ion funnel applies high-frequency voltages whose phases are inverted from each other by 180 degrees to aperture electrodes neighboring each other in an ion beam axis direction, to create a radio-frequency electric field for focusing ions.
Patent Literature 2 discloses still another type of ion transport optical system, called a “radio-frequency carpet”, in which a number of ring electrodes are formed in a substantially concentric pattern on a printed circuit. The radio-frequency carpet applies high-frequency voltages whose phases are inverted from each other by 180 degrees to concentric ring electrodes neighboring each other in a radial direction, to create a radio-frequency electric field for focusing ions.
In short, these are both ion transport optical systems using the effect of a radio-frequency electric field.
In the aforementioned mass spectrometers, neutral particles such as molecules originating from a sample component that have not been ionized in an ionization chamber, and molecules originating from a sample solvent or originating from a mobile phase in a liquid chromatograph are introduced into an intermediate vacuum chamber in the next stage of the ionization chamber together with the generated ions. Such neutral particles are not affected by the electric field, and thus when the neutral particles reach an analysis chamber and are introduced into a quadrupole mass filter, the neutral particle may not be removed by the mass filter and may reach an ion detector. The neutral particles become a significant cause of noise in the ion detector. Thus, in recent years, an off-axis structure has been employed including an intermediate vacuum chamber of low-level vacuum at a stage next to the ionization chamber, where the axis of the ion introduction port of the intermediate vacuum chamber and the axis of the ion passage opening of the intermediate vacuum chamber for sending out ions to the next intermediate vacuum chamber are offset from each other.
In the off-axis structure, the traveling direction of a stream of ions that spread out around an ion beam axis while flying needs to be bent and turned into another direction. Therefore, in general, it is difficult to ensure a high ion transmission efficiency compared with the case where ion beam axes lie on a straight line. Thus, an off-axis structure ion transport optical system has been developed that captures ions using a radio-frequency electric field while bending the traveling direction of the ions.
For example, an ion transport optical system disclosed in Patent Literature 3 has a structure in which two ion funnels each formed by arranging electrodes of substantially C-shape made by cutting out part of rings are disposed substantially in parallel to each other with the respective cut-out portions brought close to each other. Then, by appropriately setting the condition of voltages applied to the electrodes of the ion funnels, ions are offset through the cut-out portions from one of the ion funnels to the other ion funnel, whereby achieving an off-axis structure. Non-Patent Literature 1 discloses a dual ion funnel of off-axis structure in which the central axes of two, front-stage and rear-stage ion funnels are offset from each other, and the traveling direction of ions is bent inside the rear-stage ion funnel.
However, in such a conventional ion transport optical system of off-axis structure, the structure or the shape of electrodes are complicated, or the condition of voltages applied to a number of electrodes is complicated. This significantly increases the cost of the apparatus and degrades the maintainability, as compared with typical ion transport optical systems. Additionally, in a dual ion funnel, neutral particles that have to be removed collide with an electrode of the ion funnel, and thus the electrode is prone to be contaminated, which easily causes a decline in ion-transporting performance with time.